Tabular silver halide grains are grains possessing two parallel crystal faces with an aspect ratio of two or more. Said aspect ratio is defined as the ratio between the diameter of a circle having an equivalent surface area as one of these crystal faces, and the thickness, being the distance between the two major faces.
Tabular grains are known in the photographic art for quite some time. As early as 1961 Berry et al. described the preparation and growth of tabular silver bromoiodide grains in Photographic Science and Engineering, Vol 5, No 6. A discussion of tabular grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, p. 66-72.
Early patent literature includes Bogg U.S. Pat. No. 4,063,951, Lewis U.S. Pat. No. 4,067,739 and Maternaghan U.S. Pat. Nos. 4,150,994; 4,184,877 and 4,184,878. However the tabular grains described herein cannot be regarded as showing a high diameter to thickness ratio, commonly termed aspect ratio. In a number of US-A's filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are described as e.g. U.S. Pat. Nos. 4,434,226; 4,439,520; 4,425,425 and 4,425,426 and in Research Disclosure, Volume 225, January 1983, Item 22534.
For radiographic applications the main photographic advantages of tabular grains compared to normal globular grains are a high covering power at high forehardening levels as described in U.S. Pat. No. 4,414,304, a high developability and higher sharpness especially in double side coated spectrally sensitized materials thereby lowering of cross-over as specifically described in U.S. Pat. Nos. 4,425,425 and 4,425,426.
In the references on {111} tabular grains cited above especially silver bromide or silver iodobromide emulsions having a high sensitivity are disclosed, although it has been shown that high speed can also be achieved with tabular silver halide grains rich in silver chloride as in EP-A 0 678 772.
Anisotropic growth characteristics for the said tabular grains are known to be due to the formation of parallel twin planes in the nucleation step of the precipitation. However for the said {111} tabular silver halide grains rich in silver chloride use of a crystal habit modifier in relatively high amounts is therefore required, as has been illustrated in U.S. Pat. Nos. 4,713,323; 4,804,621; 5,176,692; 5,183,732; 5,185,239; 5,252,452; 5,286,621; 5,298,385 and 5,298,388. Treatment with iodide of tabular grain emulsions having {111} crystals rich in silver chloride in order to get an enhanced morphological stability and enhanced photographic performance has been disclosed in EP-A 0 678 772 and in Research Disclosure 388046, published Aug. 1, 1996.
However as a global result fairly heterogeneous emulsion crystal distributions are obtained: a common variability coefficient (defined as a ratio between average standard deviation on equivalent circular diameter and the said average equivalent circular diameter) of 0.30 to 0.60 is calculated, partly due to the presence of quite a large number of non-tabular grains having a sphere equivalent diameter of less than 0.3 .mu.m. Moreover differences in thickness growth are observed, said differences leading to unevenness as a consequence of observed differences in image tone.
Heterodispersity of grain morphology further leads to e.g. uncontrolled chemical and spectral sensitization, lower contrast and lower covering power, thereby loosing typical advantages of the said grains as referred to hereinbefore.
Until now efforts in order to get more monodisperse tabular silver halide crystal distributions in emulsion preparation have been directed towards silver halide crystals rich in silver bromide as has e.g. been described in U.S. Pat. Nos. 4,797,354; 5,147,771; 5,147,772; 5,147,773; 5,171,659; 5,248,587; 5,204,235; 5,210,013; 5,215,879; 5,250,403; 5,252,442, 5,252,453; 5,254,453; 5,318,888; 5,439,787; 5,472,837; 5,482,826 and 5,484,697 and in Research Disclosure No. 391, p. 713-723 (1996).
In order to control thickness growth for tabular {111} grains rich in silver chloride a solution has been proposed as disclosed in EP-A 0 866 362: therein a preparation method has been given wherein the dispersion medium during nucleation is held constant at an initial pH value between 6.0 and 9.0; further setting pH to a value lower than 6.0 for at least 30 seconds, between ending the said nucleation step and ending the said growing step; followed by resetting pH to the said initial pH value.
Nevertheless the problem remains of too low a covering power, probably due to the presence of still a large number of grains showing a higher thickness than the average thickness observed.
As a consequence many attempts have been made in order to improve the degree of homogeneity of the size and shape of the crystals but the majority of them is related with tabular grains rich in silver bromide again. So radiographic materials comprising emulsions having monodisperse tabular silver brom(oiod)ide crystals have e.g. been described in U.S. Pat. Nos. 5,252,442 and 5,508,158. The same preparation methods as for the forementioned tabular grains rich in silver bromide can however not be applied as such in preparing tabular grains rich in silver chloride, especially due to the presence of crystal habit modifiers, usually adenine, as this leads to the disadvantages set forth hereinbefore.